What is causing a persistent knocking noise whenever I step on the brake?

Hello, Les Schwab is built around tire changes, brakes, alignments, shocks, and a few other maintenance items. For non-maintenance items, you should be seeking help from an ASE certified service center, of which there are many certified by RepairPal. You may be hearing the fan contacting the upper radiator hose or fan shroud when you are hitting the brakes. If the motor mounts and transmission mount is severely worn, the motor may be lurching forward under force of inertia when you press the brakes. Have this inspected, as the clunk when shifting to drive is likely the motor mounts and transmission mounts. See here for certified facilities in your area: http://repairpal.com/gmc-within-10-miles-of-bremerton-washington-98337